In order to reduce noxious exhaust gas emissions from a vehicle, fuel injection is controlled on the basis of output signals from an oxygen sensor (O2 sensor). For example, when the output voltage of the oxygen sensor is low (i.e., when an air/fuel ratio is lean), P-jump delay time for fuel injection control is increased such that more fuel is injected into the engine.
For normal operation of an oxygen sensor, the temperature of the oxygen sensor should be maintained at an appropriate activation temperature. In the case where the activation temperature is not maintained by the exhaust gas from the engine (e.g., just after the engine has been started), the oxygen sensor is actively heated. During such controlled heating of the oxygen sensor, an electric heater provided in the oxygen sensor is duty controlled.
The duty (hereinafter referred to as “heating duty”) for controlling the heating of the oxygen sensor is obtained by multiplying a feedback factor (hereinafter referred to as a “heating factor”) to a base duty calculated on the basis of engine speed and engine load. That is, the heating factor is P-I (proportionally-integrationally) controlled on the basis of the output voltage of the oxygen sensor, so that the oxygen sensor is feedback controlled.
According to the prior art, such a heating factor is only increased or decreased simply on the basis of the output voltage of the oxygen sensor. However, when an oxygen sensor is aged (e.g., heat-aged), it may produce a malfunction that can not be compensated by the prior art. One typical example of such an oxygen sensor malfunction is a switching time error, in which the switching time needed for the sensor to detect a transition from a lean to a rich A/F ratio, or vice versa, exceeds a threshold switching time. A switching time index is calculated as the ratio of the switching time to the threshold switching time, and the occurrence of a switching time error is determined on the basis of whether the switching time index is greater than a predetermined threshold index.
Another example of an oxygen sensor malfunction is a frequency characteristic error in which the frequency characteristic of the output voltage of the oxygen sensor does not reach a threshold frequency. A frequency index is calculated as being proportional to the reciprocal of the frequency, and the frequency characteristic error is determined on the basis of whether the frequency index is greater than a predetermined threshold frequency index. For example, according to non-opened experiments and regarding aged oxygen sensors of a Titania type, the switching time index and the frequency index have been found to became very high while the heating factor is controlled low. In addition, in this case, the P-jump delay time has been found to become a very large value. That is, when an oxygen sensor is heat-aged, the switching time index and the frequency index may become deteriorated at the same time that excessive fuel injection occurs. However, according to a heating control of an oxygen sensor of the prior art, normal operation of the oxygen sensor is premised such that only minimal heating of the oxygen sensor is performed.